fix for bug#8461

  BUG 8461 - TRUNCATE returns incorrect result if 2nd argument is negative
  Reason: Both TRUNCATE/ROUND converts INTEGERS to DOUBLE and back to INTEGERS
  Changed the integer routine to work on integers only.
  This bug affects 4.1, 5.0 and 5.1
  Fixing in 4.1 will need to change the routine to handle different types individually.
  5.0 did had different routines for different types already just the INTEGER routine was bad.


sql/item_func.cc:
  fix for bug#8461
  BUG 8461 - TRUNCATE returns incorrect result if 2nd argument is negative
  Reason: TRUNCATE converts INTEGERS to DOUBLE and back to INTEGERS
  Both ROUND and TRUNCATE are affected by this.
  Changed the integer routine to work on integers only.
  This bug affects 4.1 5,0 and 5
  Fixing in 4.1 will need to change the routine to handle different types individually.